Invasive Plants

 

Managing Invasive Plants at Pinnacles National Park

Out of approximately 625 plant species at Pinnacles National Park, about 100 are nonnative. Nonnative species that have the potential to cause serious ecological harm by displacing native plant communities are classified as invasive. The Pinnacles National Park Weed Control Program primarily targets the following invasive species:

• Yellow star thistle (Centaurea solstitialis)
• Italian thistle (Carduus pycnocephalus)
• Horehound (Marrubium vulgare)
• Mustard (Hirschfeldia incana)

Focused Weed Control Efforts

The program focuses on these species due to their significant potential to destroy native habitats. Yellow star thistle and mustard are managed by treating large areas on a monthly basis. Horehound, which is nearing eradication within the park, is controlled through monthly visits to approximately 140 small plots.

Eradication Methods

Control methods include hand pulling and herbicide application, ensuring the most effective approach to protecting and restoring native habitats at Pinnacles.

 

Yellow Star Thistle has stiff spines that make it difficult to manage.

NPS

Yellow star thistle
(Centaurea solstitialis)

Yellow star thistle, or “the plant that ate California,” has already degraded over 25% of the state's land and is considered one of the most invasive weeds by the California Exotic Pest Plant Council. Yellow star thistle is a summer-blooming annual in the sunflower family and native to Eurasia. The plant is often initially found in open, disturbed areas such as former ranch lands, road edges and stream channels. Through time it moves increasingly into undisturbed locations, including meadows and riparian corridors.

Yellow star thistle produces a taproot that extends deeper than other similar annual species. This allows yellow star thistle to grow well into the summer after most other annuals have dried up. Each seed head produces stiff 1-3 cm long spines that make the plant unpalatable to wildlife and painful for park visitors. Yellow star thistle imposes a serious long-term threat because of its ability to produce large numbers of seeds and its growth during the hot summer months.

 

Purple flowers and a thorny stem help identify Italian thistle.

NPS

Italian thistle
(Carduus pycnocephalus)

Native to the Mediterranean and middle east, Italian thistle was accidentally introduced to California, but is now widespread in much of the state. Plants are tall (up to 7 feet) and slender, with spines on the leaves, stems, and flowerheads. Flowers are purple and occur in clusters. Italian thistle produces two types of seeds: white seeds, which are wind dispersed, and brown seeds, which eventually fall to the ground beneath the plant. Seeds turn sticky when wet, which also helps them disperse. Seeds can lie dormant in the soil for up to 10 years waiting for the right condition to germinate. These qualities allow Italian thistle to reproduce readily, persist in different environmental conditions, and overtake native plant communities.

Italian thistle is not as well established at Pinnacles, but its preferred habitat along creeks and valley bottoms is some of the most important in Pinnacles for wildlife. Its limited quantity and potential to impact important habitat make it a good candidate for removal.

 

Small white flowers and leaves with tiny woolly hairs distinguish horehound.

NPS

Horehound
(Marrubium vulgare)

Native to Europe, horehound arrived in North America as a cultivated herb. It reproduces aggressively by both seed and vegetative means. Recurved barbs on the seed can easily attach to fur and clothing, making it easy for wildlife and visitors alike to help it disperse. Horehound infestations are often located at corral sites and animal burrows, pointing to animal fur (possibly the fetlocks of horses and the fur of small animals) as vehicle for transporting horehound seeds at Pinnacles. The park staff has been successfully controlling horehound since the late 1980s, and it is nearly eradicated in the core of the park. However, horehound is still common around the campground, where frequent soil disturbances create good conditions for this species.

 

Summer Mustard has small yellow flowers when it blooms in the summer.

NPS

Summer Mustard
(Hirschfeldia incana)

Summer mustard is a biennial (grows to flower every two years) native to the Mediterranean. The plant was first established in southern coastal California and now can be found in Oregon, Idaho, Nevada, and Washington. Summer mustard grows extremely well in disturbed, open, and sunny areas. In the first year of growth, the mustard plant produces a rosette; in the second year, the plant produces a flowering stem, sets seed, and dies. The plant blooms May through October. In late fall, it forms dense stands of brittle woody seed stalks. At Pinnacles, summer mustard is out-competing native plant species, encroaching on trails, and the dried seed stalks are creating hazardous fuel buildup.